For the benefit of others as slow as myself, here's what you do:
Right click on the monitor, select Properties, and under Settings, find the dimensions of your screen. Open up your favorite drawing program (I use Adobe Photoshop Elements.) and create a new image using the size of your screen. Construct your shapes. I prefer Mondrian, but there's no reason you can't have diagonals, stars, triangles, anything you like. Save the image as a .jpg file.
Go to the Desktop tab on your monitor properties and set the .jpg file as
background image. That's it. You're done.
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